Frequency modulated continuous wave (FCMW) radar can be used to determine the range to a target. In contrast to other radar systems, such as pulsed radar, FCMW radar systems radiate continuous transmission power. The frequency of the transmitted radar beam is changed with time, in a frequency chirp. The bandwidth of the chirp, i.e. the difference between the lowest and highest transmitted frequencies, determines the resolution of the radar system. Systems with larger bandwidths have higher resolutions than those with smaller bandwidths. Thus to detect nearby targets, a large bandwidth is needed, for example 1 GHz or 4 GHz for automotive radar systems.
A frequency chirp has a defined rise and fall time, often called acquisition time and reset time respectively. All the important information for determining range is measured during the acquisition time. The reset time is the time needed to reset the chirp from its final frequency to its initial frequency. It is desirable that the reset time be as short as possible, to maximise useful acquisition time, leading to a better dynamic range of the radar system and lower power consumption (due to shorter data post-processing times). Thus the rate of change of frequency is typically much higher in the reset time than in the acquisition time.
The frequency chirps for FCMW radar systems are typically generated by a phase locked loop (PLL). For such closed loops, faster reset times require higher PLL bandwidths. However, higher PLL bandwidths also result in higher phase noise, which is not desirable for radar systems. This problem is further compounded by large chirp bandwidths, which increase the rate at which the frequency must be reset due to the larger difference between the initial and final frequencies.